This invention relates to microdevices, and more particularly to pneumatic actuators and micro-valves.
Microdevices, such as microfluidic control devices and micromachines, are used in a wide variety of modern devices. Currently, microdevices are used in automobiles, medical instrumentation, or process control applications, and in conjunction with appropriate sensors can provide accurate determinations of pressure, temperature, acceleration, gas concentration, and many other physical or chemical states. Microfluidic control devices include micro-valves for handling gases or liquids, flow gauges, and ink jet nozzles, while micromachines include micro-actuators, movable micro-mirror systems, or even tactile moving assemblies.
Large arrays of micro-valves have particular utility in conjunction with air jet paper transport systems or other material processing systems that must precisely control position and velocity of paper or other objects moving through the system. Commonly, material processing systems control object movement by physically engaging the object with a separate object drive mechanism that moves the object at a predetermined velocity along a predetermined path. For example, gear driven ratchets, rollers, hooks, or conveyors are widely employed to move objects as diverse as paper, semiconductors, plastics, or steel by mechanically engaging the objects, and moving the engaged objects along a desired path at a fixed velocity. While commonplace, mechanical or frictional engagement of objects does have a disadvantage of requiring direct physical contact with an object. In contrast to mechanical or frictional transport systems, object drive mechanisms based on various fluid support techniques have long been employed to move delicate objects without requiring solid mechanical contact. For example, instead of using conventional belts, conveyors or rollers, paper moving through xerographic copier systems can be supported on a laminar air flow, or uplifted and moved by directed air jets. This form of fluid support is particularly advantageous, for example, when sheets of paper carrying unfixed toner images must be moved between a photoconductive drum and a fusing station where the toner image is fixed.
One type of micro-valve used in air jet systems is an electrostatic flap valve, which controls the flow of air passing through a port (orifice) formed in a pressure wall separating a high pressure air source and a paper transport passage. Each electrostatic flap valve typically includes a fixed electrode mounted on the downstream surface of the pressure wall surrounding the port, and a flap member including a flexible electrode that is attached at one end to the pressure wall. Flow through the flap valve is controlled by applying a suitable potential to the fixed and flexible electrodes. To open the flap, thereby allowing fluid to flow from the high-pressure source to the transport passage through the orifice, the potential is removed (turned off), allowing the pressure differential to push the flap open. To subsequently close the flap, the potential is applied (turned on), thereby causing electrostatic attraction between the fixed and flexible electrodes to pull the flap against the pressure wall to close the orifice. One advantage of electrostatic flap valves is that significant power is expended only during valve opening or closing. That is, when flap valves are in an open state or in a closed state, no current flows to maintain either state. Only displacement current flows during valve state transition between the opened and closed states.
A problem with the use of electrostatic flap valves to control fluid flow between high and low pressure regions is that a large force, and therefore a high voltage potential, is required to close the flap against the flow passing through the orifice between the high and low pressure regions. Once the flap is closed, the fixed and flexible electrodes are in very close proximity, and the voltage potential needed to maintain the closed state is relatively small, essentially because the electrostatic force is inversely proportional to the square of the distance between electrodes. However, when the flap is opened, the fixed and flexible electrodes are far apart, and a substantially larger voltage potential is needed to pull the flap closed. Further, the fluid flowing through the orifice applies a force against the flap that further increases the needed voltage potential, and if large enough, this force can prevent closure even when extremely high voltages are used. These voltages are ultimately limited by breakdown mechanisms between the fixed and flexible electrodes. Accordingly, when electrostatic flap valves are utilized in this manner, the pressure gradient across the high and low-pressure regions is limited by the available voltage potential to allow closure of the flap.
Microdevice actuators often include micromachined monocrystalline structures or piezoelectric devices to perform a desired operation, such as to position a micro-mirror in a fiber-optic micro-switch. However, both monocrystalline structures and piezoelectric devices are relatively expensive to produce, and require relatively high voltage sources to produce required positioning forces.
What is needed is a cost effective pneumatic valve and a cost effective pneumatic actuator that can be driven using low voltages and low power. What is also needed is an inexpensive pneumatic valve for controlling high-pressure fluid flows.
The present invention is directed to a low-cost pneumatic actuator that facilitates both low-power micro-valve and low-power micro-actuator functions by utilizing a pair of electrostatic flap valves to control the flow of a fluid into an expandable chamber such that the electrostatic flap valves are only closed under equilibrium (i.e., zero flow) conditions or approximate thereto (e.g., zero to 10% of the full flow rate) such that minimal force is needed to close the valves.
The pneumatic actuator includes a housing defining a cavity, an elastomeric membrane (e.g., a silicone film) mounted over the cavity to form an expandable chamber, and a pair of electrostatic flap valves for controlling fluid flow into and out of the expandable chamber. According to an aspect of the present invention, a first electrostatic flap valve is mounted inside the expandable chamber such that it is positioned to selectively block fluid flow into the chamber through a first orifice, and a second electrostatic flap valve is mounted outside the expandable chamber such that it is positioned to selectively block fluid flow out of the chamber through a second orifice. In one embodiment, the first orifice is connected to a high-pressure source, and the second orifice is connected to a low-pressure source (e.g., to the external atmosphere through a vent hole formed in an upper wall of the housing).
During operation, the elastomeric membrane is distended (stretched) by releasing the first electrostatic flap valve (i.e., causing an associated control circuit to turn off the control signals transmitted to the first flap valve) while maintaining the second electrostatic flap valve in a closed position, thereby allowing the high pressure gradient across the first orifice to push the first flap valve open. When the pressure gradient across the electrostatic membrane is balanced by the elastic restoring force of the distended membrane, flow into the cavity stops. It then requires negligible electrostatic force to close the first flap valve. Conversely, the elastomeric membrane is subsequently collapsed by releasing the second electrostatic flap valve while maintaining the first electrostatic flap valve in a closed position, thereby allowing the high pressure inside the expandable chamber to escape through the second orifice. According to the present invention, the first electrostatic flap valve is only actuated to block the first orifice when an internal pressure of the expandable chamber is approximately equal to the pressure supplied by the high pressure source, and the second electrostatic flap valve is only actuated to block the second orifice when the internal pressure of the expandable chamber is approximately equal to the exhaust pressure (for example, atmospheric pressure). As described above, conventional arrangements require at least one valve to close against the flow of gas through the orifice, which requires a voltage much higher than that needed to hold off the static pressure. Unlike these conventional arrangements, neither flap valve of the pneumatic actuator is required to close against the flow of fluid. Accordingly, the present invention provides a mechanism for repeatedly expanding and contracting an expandable chamber that requires voltages sufficient only to hold off the pressure gradient. Alternatively, the same voltage may be used as for conventional arrangements, but much higher pressures can be utilized.
According to an embodiment of the present invention, an array of pneumatic actuators are formed on a housing that is fabricated using substantially conventional printed circuit board fabrication techniques. By utilizing the conventional printed circuit board fabrication techniques to construct the housing, the manufacturing costs associated with the production and electronic driving of the pneumatic actuator array are significantly lower than the costs associated with the production of conventional micro-actuator arrays.
According to another embodiment of the present invention, the pneumatic actuator array of the present invention is utilized as a fluid control valve array in which the distendable portions of the elastomeric membrane are positioned to selectively block, for example, air jet ports in an air jet paper (or other object) transport system. This arrangement overcomes problems associated with conventional air jet arrays that utilize electrostatic flap valves to directly open and close associated air jets. In particular, unlike conventional air jets in which the flap valve must close against the flow of air, the electrostatic flap valves of each pneumatic actuator open and close when pressure inside the distended membrane is equalized. Therefore, the limitation on conventional air jet flows, which is limited by the ability of the flap valves to close off through their flow fields, is eliminated in paper handling systems that incorporate the pneumatic actuators of the present invention. Accordingly, a much higher paper transport air pressure may be provided than that usable in conventional air jet paper handling systems, which facilitates improved control over paper movement through the system.
According to yet another embodiment of the present invention, the pneumatic actuator of the present invention is utilized as a micromachine to control the position or orientation of an object, such as a micro-mirror structure in a fiber-optic micro-switch, thereby providing a low cost, low voltage, low-power alternative to conventional actuators.